1. Field of the Invention
The present invention relates to a transmitter-receiver of a wireless communication system. In particular, the present invention relates to cancellation of passive intermodulation interferences in a transmitter-receiver of a wireless communication system.
2. Discussion of the Related Art
In a wireless base station, the sensitivity of an uplink receiver can be severely degraded by undesirable interfering signals within the uplink frequency band. FIG. 1 is a block diagram of base-station station 100 in a wireless communication system using frequency-division-duplexed (FDD), with antenna 101 being shared between downlink transmitter 102 and uplink receiver 103. As shown in FIG. 1, duplexer 104 is a three-port radio-frequency (RF) filter that isolates reverse-direction receiver signal 110 from forward-direction transmitter signal 111. Forward-direction transmitter signal 111, which is provided by power amplifier (PA) 105, is typically a high-power RF signal. From duplexer 104, transmitter signal 101 is sent into free space by antenna 101. Nonlinear junctions in degraded antenna components (e.g. connectors) and rusty objects near antenna 101 (e.g. metal fences) introduce passive-intermodulation (PIM) interference in the reverse direction (i.e., back to receiver 102). The PIM effect is thus also known as “the rusty-bolt effect” and has been recognized by the wireless industry as a difficult and complicated problem.
PIM interferences may be detected during antenna installation with currently available instruments using a high-power, two-tone signal. However, PIM problems often surface as a result of gradual degradation only years after installation. Carrying out instrument-based PIM test interrupts the service. Furthermore, when a base station has begun service, an instrument test may not be allowable because the PIM test sends a high-power RF signal into free space in a frequency band that is not licensed to the service provider.